Techniques for controlling current to drive a multiphase rotating electric machine by part of pulse width modulation (PWM) have been known in the related art. For example, if the multiphase rotating electric machine is a three-phase motor, a PWM reference signal of a triangular wave or the like is compared with a voltage reference signal related to voltages applied respectively to its three-phase windings and current flowing through the three-phase motor is controlled by switching on and off switching elements of an inverter.
If the inverter is connected to a capacitor, when no current flow into the inverter, the capacitor is charged as current flows from a power supply source into the capacitor. On the other hand, when current flows into the inverter, the capacitor is discharged as current flows from the capacitor into the inverter. In PWM control, the capacitor alternates between charging and discharging during one cycle of PWM, capacitor current is pulsed. Pulsation of current flowing through the capacitor is referred to as ripple current. When the capacitor current is pulsed, noises are generated or the capacitor generates heat. In addition, fluctuation of a voltage applied to the inverter may result in poor controllability of inverter current.
For the purpose of avoiding the above problems, JP 2001-197779A discloses a technique in which a phase difference is imposed on switching timings of switching elements between two sets of bridge circuits, based on pre-stored map data, so that a waveform of summed capacitor current approaches a smooth waveform in order to decrease ripple current. In addition, JP 2007-306705A discloses a technique in which, if two axes are connected in a PWM amplifier, a voltage command for one axis is biased to Vcc/4 (Vcc being a power source voltage) while a voltage command for the other is biased to −Vcc/4 in order to decrease ripple current.
However, the technique disclosed in JP 2001-197779A requires a delay circuit since the phase difference is imposed on the switching timings based on a modulation ratio and a power factor. In addition, this technique requires detection of current in a plurality of lines at short intervals, which may result in heavy operation load of a control circuit.
In the technique disclosed in JP 2007-306705A, for example if two inverter systems are present, a voltage command is biased to a ¼ upper part of a power source voltage for one of the two inverter systems. When the command voltage is biased upwards (higher), time for which a switching element at a higher potential is in the on-state is longer than time for which a switching element at a lower potential is in the on-state. On the other hand, for the other inverter system, the voltage command is biased to a ¼ lower part of the power source voltage. When the command voltage is biased downwards (lower), time for which a switching element at a lower potential is in the on-state is longer than time for which a switching element at a higher potential is in the on-state. If an integrated value of current flowing through the switching elements at the higher potential is significantly different from an integrated value of current flowing through the switching elements at the lower potential, it may result in a difference in heat loss between the switching elements at the higher potential and the switching elements at the lower potential. Such a difference in heat loss between the switching elements at the higher potential and the switching elements at the lower potential requires a marginal thermal design or an asymmetrical heat radiation design. In addition, this difference in heat loss may require additional elements in the switching elements at the higher potential and the switching elements at the lower potential, which may result in cost-up.